Calcitonin (CT) is a polypeptide hormone with potent hypocalcemic effects. Detailed information is lacking concerning the biosynthesis of this hormone. We plan to study the molecular events involved in the synthesis of CT and factors that regulate CT gene expression. Using isolated messenger RNA (mRNA) we have synthesized and cloned double stranded calcitonin complementary DNA (cDNA) in bacterial hosts. The recombinant DNA molecule will be emplified in E. Coli x1776, cDNA inserts released by PsT-I cleavage and sequenced by the technique of Maxam and Gilbert. The calcitonin cDNA sequence will elucidate the structure of the calcitonin precursor and also provide structural information on the non-coding regions of the calcitonin messenger RNA. The cloned calcitonin DNA will be labeled with 32P of high specific activity by the nick-translation procedure and used as a probe to explore a number of biological questions. These include (1) the organization of calcitonin genomic sequences by filter hybridization assays, (2) the measurements of messenger RNA levels in rat thyroids by hybridization assays to define the nature of calcitonin gene regulation in hyper- and hypo-calcemic states, and (3) the presence of calcitonin messenger RNA, as detected by hybridization techniques in extra thyroidal tissue including pituitary and brain, where immunoreactive calcitonin has recently been discovered.